With the advent of computerized tomography and the availability of a safer water-soluble contrast medium (Metrizamide) for studies of the subarachnoid space, new and more reliable techniques are available for the study of the distal absorptive pathways of cerebrospinal fluid (CSF) in man and experimental animals. The present studies are designed to improve understanding of the morphology, pathophysiology and radiology of obstructions or other alterations of the arachnoid granulations and distal subarachnoid pathways. These studies include: (1) the passage of Metrizamide through arachnoid granulations; (2) the development of arachnoid proliferations (villi and granulations) and distal CSF absorptive pathways in human and rabbit; (3) pathways of CSF absorption in a lower vertebrate (dogfish); Elasmobranchii have no extraventricular CSF and it is therefore of interest to compare their absorptive pathways with those of man; (4) the passage of blood through the arachnoid proliferations of rabbit and cat with and without anticoagulants; and (5) radiologic and morphologic observations of the arachnoid granulations and adjacent structures in human autopsy material with hydrocephalus and related diseases. Radiological, physiological and morphological techniques will be used for these studies.